This invention relates to sensing circuits using MOS transistors and more particularly to a circuit for detecting low-level signals as may be present at the output of a read-only-memory or a keyboard input to a microcomputer chip.
In an MOS/LSI chip of the type shown in U.S. Pat. No. 3,988,604, issued Oct. 26, 1976 to J. H. Raymond and assigned to Texas Instruments, a read-only-memory or ROM is disclosed which is of the virtual ground type. A similar ROM is disclosed in U.S. Pat. No. 3,900,722, issued Aug. 17, 1975 to Michael J. Cochran and Charles P. Grant, Jr., also assigned to Texas Instruments. In these devices, the ROM output lines are precharged during one part of an operating cycle, then conditionally discharged during a read cycle when the ROM output is detected. The ROM output lines are elongated diffused regions in the semiconductor chip and can exhibit large capacitance. As the number of bits on an output line is increased, the capacitance increases because the line becomes longer. This causes the decay time to be longer, so the precharged output lines may not decay to a definite "ground" logic level during the alloted time in the read cycle. This problem is more troublesome as the size of the ROM is increased from say 8K to 16K to 32K, etc., as the number of bits on a line may go from 64 to 128 to 256. A similar problem of detecting a voltage change which is less than a logic level occurs in many situations in MOS/LSI circuit devices. For example, a keyboard input to a calculator chip may be less than a full logic level of 0 to 9v.
It is therefore the principal object of this invention to provide a sensing or detecting circuit which will respond to a low level signal in a short time period. Another object is to provide an improved detector circuit for a ROM output or a keyboard input (or similar circuit) in an MOS/LSI device. A further object is to provide a simple and small circuit layout for a sense or detector circuit.